1. Field of the Invention
This invention relates generally to a sheet registration device, and more particularly, to a pivoting deskew stalled roll registration system.
2. Description of Related Art
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
In printing machines such as those described above, it is necessary to align and register the individual cut sheet so that the developed image is placed in the proper location on the sheet. Various schemes have been developed to assure that the image-receiving sheet is in the proper location and forwarded at the proper time. Some complex printing machines utilize various sensors and translating nips to align the sheet in the proper position for receiving the image. Other machines utilize variable speed stepping motors to differentially drive a sheet within a sheet path for deskew and registration purposes. Both of these registration methods require sophisticated control and are relatively high cost.
Another method for registering and aligning a sheet is the use of stalled rolls. In the stalled roll technique, a sheet is driven into a nip in which the rolls are stopped causing a buckle to be formed between the stalled roll and the driving rolls. The force of the buckle causes the lead edge of the sheet to align itself within the stalled nip and the stalled nip is then activated so that the sheet is forwarded in the proper aligned position. Other systems utilize a stall roll with a solenoid actuated drive nip in which the drive nip precedes the stalled roll so that the sheet is free to deskew in the stalled nip. While simpler than the active registrations described previously, the stalled roll technique with solenoid actuated nip still requires a solenoid to deactivate the drive nip. Other problems arise if the buckle in a stalled roll system gets too large which can then cause the registration force to decrease and the lead edge of the sheet to back out of the nip causing skew.
It is desirable to have a stalled roll registration device in which a sheet could be deskewed and registered within the stalled nip and then secured prior to being forwarded in timed registration to a subsequent machine subsystem.
In U.S. Pat. No. 5,235,862 to Acquaviva et al., issued Oct. 19, 1993 a sheet handler is disclosed that includes an idler and driven cross roller set. The rollers are preloaded so that a normal force exists between the rollers at the nip. The nip is provided with an apparatus for adjusting the preloaded force to adjust the normal force on the sheet material passing through the nip.
A method and apparatus for deskewing and registering a sheet in a short paper path is shown in U.S. Pat. No. 5,156,391 issued Oct. 20, 1992 to Roller, by differentially driving two sets of rolls so as to create a paper buckle buffer zone in the sheet and then differentially driving a roll set to correct skew while the sheet is still within the nips of multiple drive roll sets.
U.S. Pat. No. 5,078,384 issued Jan. 7, 1992 to Moore discloses a method and apparatus for deskewing and registering a sheet, including the use of two or more selectably controllable drive rolls operating in conjunction with sheet skew and lead edge sensors for frictionally driving and deskewing sheets having variable lengths. Sheets will be advanced to reach a predetermined registration position at a predetermined velocity and time at which time the sheets will no longer be frictionally engaged by the drive rolls.
A two step optimized stalled roll registration and deskew system is shown in U.S. Pat. No. 5,775,690 issued Jul. 7, 1998 that includes a drive mechanism preceding a stalled roll pair and a sensor to determine the size of a buckle formed in a sheet as it is fed into the registration nip formed by the stalled roll pair. When the buckle reaches a predetermined size the sensor generates a signal which causes the drive controller to briefly pulse the registration roll pair. This brief pulse of the registration roll pair captures the sheet in the nip in a deskew and registered position for subsequent feeding in a timed relationship to a machine subsystem. A baffle located between the drive nip and registration nip directs the sheet buckle formation in a controlled manner so that proper deskewing and registration forces are obtained.
U.S. Pat. No. 5,632,478, issued May 27, 1997 to Lisbeth S. Quesnel describes a stalled roll registration device in which there is provided a drive mechanism preceding the stalled roll which allows a sheet to move while within the drive nip. The drive mechanism uses a drive roll and an eccentric idler roll in contact therewith. The idler is biased against the drive roll by a compression spring such that as the eccentric idler roll rotates, the spring is alternately compressed and relaxed. When a sheet is driven through the drive mechanism and into the stalled nip, a buckle is formed which causes a force to be exerted on the drive nip, which causes the eccentric roll to stall in the horizontal position in which little normal force is exerted on the sheet. The sheet is then free to deskew and align in the stalled nip.
Even though the above-mentioned registration and deskewing systems are useful, there is still a need to remove large amounts of sheet input skew that cannot be removed by the standard stalled roll system.
Accordingly, pursuant to the features of the present invention, an improved stalled roll registration and deskew system is disclosed that answers the above-mentioned problem by providing a hard roller registration nip in conjunction with a ball-on-belt sheet transport. The ball-on-belt transport facilitates rotational movement of a sheet against the registration nip as the sheet is driven into the nip.
These and other features and advantages of the invention are described in or apparent from the following detailed description of the exemplary embodiments.